Monolithic prioritiser provides low standby for back up
A dual input monolithic power prioritiser for 1.8 to 18V systems has been announced by Linear Technology. The LTC4420 normally powers the load from the higher priority main supply such as a wall adapter or battery, switching to the backup supply—typically a battery or large-value capacitor—during primary brownout or power loss conditions. With up to 18V operating capability, it accommodates a range of power sources such as wall adapters, USB ports, super-capacitors, and stacked batteries with Li-Ion, alkaline or NiMH cells.
By reducing device operating current to 3.6microA and the backup supply’s standby current drain to a low 0.32microA, the prioritiser extends run time in portable battery-operated systems. It does not require the backup supply voltage to be lower than the main supply voltage. When the backup supply voltage is higher than the main supply voltage, a prioritiser does not necessarily draw current from the highest voltage supply. Internal 500mA (min) current-limited switches multiplex from the primary to the backup supply when the primary voltage falls below the adjustable switchover threshold. Break-before-make fast switch control blocks reverse and cross conduction currents while minimising output voltage droop. To reduce quiescent current, the backup supply’s resistive divider is connected to a strobed ground with a duty cycle that is adjustable down to a low 0.0015 per cent, and an optional under-voltage disconnect prevents deeply discharged batteries. A freshness seal mode further lowers standby current to 0.12microA, minimising battery discharge during product storage and shipment. Reversed-battery voltages up to -15V are blocked from propagating to the load.
Operation is over the 0 to 70°C commercial and -40 to +85°C industrial temperature ranges. The prioritiser is offered in thermally enhanced 12pin MSOP and 3.0 x 3.0mm DFN packages. 1
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